The present invention generally related to a device and method for testing physical properties of materials. In particular, the present invention relates to devices and methods for testing the peel strength between adjacent layers of a laminate.
In the manufacturing of many products it is desirable to test the peel strength between adjacent layers of a laminate. In particular, manufacturers of refrigerated dough containers test the peel strength between layers of their multilayer containers. This type of container generally has a cylindrical, tubular sidewall that includes an inner foil layer, an intermediate cardboard layer and an outer foil-paper composite layer. The inner foil layer provides an airtight envelope and a contact surface for the dough. The cardboard layer, which is relatively stiff, generally gives the container its cylindrical shape. The composite layer provides the container with an opening means and typically serves as a product label as well.
In the above-mentioned dough containers, the inner foil layer is adhesively bonded to the inner surface of the cardboard layer. The helical joint between adjacent edges of the inner foil strip lies substantially along the helical joint in the cardboard and is hermetically sealed. The composite layer comprises an inner paper layer bonded to an outer foil layer. The paper layer is adhesively bonded to the outer surface of the cardboard layer. The joint formed between adjacent edges of the composite strip is offset from the joints in the cardboard and foil layers.
When the container is filled with dough and sealed, it is under internal pressure. The offset between the joint in the composite layer and the joints in the cardboard and inner foil layers gives the container strength to resist the internal pressure. The adjacent edges of the cardboard strips are not fastened together and the joint in the inner foil layer is too weak to contain the internal pressure. Without the composite layer, the container would burst open along the joints in the those layers. To open the container, a user peels a strip of the outer composite layer from the container along the underlying joints so that they split apart and allow access to the dough within the container.
The bursting strength of the container is partly dependent on the peel strength between the composite and cardboard layers. Also, the thickness of the composite layer is dependent on the peel strength. If the bond between the layers is too strong, the composite will fail in tension before it can be peeled from the cardboard. Therefore, in order to determine a proper structural design, container manufacturers desire to test the peel strength between the layers. Manufacturers also want to test the peel strength in order to determine the force that a consumer must exert to open the container. If the containers are difficult to open, consumer will avoid buying a product sold in the container. In addition, manufacturers test the peel strength in order to determine production quality and to determine the optimal materials for constructing the containers.
Conventional peel testing of refrigerated dough containers includes clamping a sample, typically a one-inch length of a container, directly into a lower clamp on a conventional constant-rate-of-extension testing machine. A starting portion of the composite layer is peeled from the cardboard layer near the bottom of the sample and clamped into an upper clamp on the testing machine. The testing machine is started, and the composite layer is peeled from the cardboard layer while the peeling force is measured and recorded.
One shortcoming of this procedure is that the peeling force results are not linear because the peeling angle between the outer layer and the inner layer changes during the test. This is because the sample is held fixed in relation to the lower clamp, and the composite layer is peeled from the bottom of the sample to its top. Since the outer layer is pulled from above, the peeling angle, which is defined as the angle formed between the direction of pull at the point where the peeled portion joins the unpeeled portion of the composite layer and the tangent to the outer surface of the cardboard layer at that point, varies from 0xc2x0 to 90xc2x0 during the test. In addition, until the location of the peeling point moves above a horizontal plane through the diameter of the sample, the peeled portion slides against the unpeeled portion of the composite layer, further distorting the test results. A second shortcoming is that the sample deforms as the composite layer is being peeled. The sidewall of the unpresurized sample is relatively thin and flexible and deflect from its cylindrical shape while the composite layer is pulled from bottom to top. This deflection causes additional non-linearity in the measurements.
Testing devices for testing the peel strength between layers of a cylindrical, tubular laminate which maintain a constant peeling angle are known. However, these devices are generally either custom testing machines which are costly to manufacture or devices that are not usable with tubular cardboard containers having low axial compressive strengths and/or are lightweight. Also, the custom machines are not readily adaptable to tubular laminates of different sizes and shapes.
The present invention includes an assembly for retaining a tubular laminate while an outer layer of the laminate is pulled from an inner layer of the laminate. The assembly includes a mandrel which has a proximal end, a distal end, a longitudinal axis and a retaining surface. The retaining surface is located between the proximal end and the distal end and is substantially concentric with the longitudinal axis. The retaining surface has an outer periphery shaped and sized to frictionally engage at least a portion of the inner surface of the laminate. The assembly also includes a mounting bracket for mounting the mandrel to a mount. The mounting bracket has a radial leg which has a proximal end located substantially along the longitudinal axis adjacent the distal end of the mandrel. The radial leg extends radially from the longitudinal axis and terminates at a distal end. The mounting bracket also has means for attaching the distal end of the radial leg to the mount. The assembly further includes means for rotatably mounting the mandrel to the proximal end of the radial leg of the mounting bracket.